Fengguang Lou

Influence of Al3+ and P5+ ion contents on the valence state of Yb3+ ions and the dispersion effect of Al3+ and P5+ ions on Yb3+ ions in silica glass

Three series of Yb3+-doped silica glasses containing different amounts of Al2O3 and P2O5 were prepared successfully by using a sol–gel method. Absorption, excitation and fluorescence spectra of Yb2+ ions in these silica glasses as well as X-ray photoelectron spectra (XPS) of Yb4d were recorded and analysed systematically. It is found out that the addition of Al3+ or P5+ ions has great influence on the redox state of ytterbium ions. With increasing Al3+ ion contents in these silica glasses, more trivalent Yb3+ ions are reduced to divalent Yb2+. In contrast, the increase of P5+ ion contents greatly promotes divalent Yb2+ ions to be oxidized to trivalent Yb3+. The possible redox mechanisms have been explored and discussed in detail. The influence of Al3+ and P5+ ion contents on the near-infrared luminescence intensity of Yb3+ ions and cooperative luminescence of Yb3+ ion pairs was also discussed. Both the near-infrared luminescence intensity of Yb3+ ions and cooperative luminescence of Yb3+ ion pairs decrease gradually with increasing Al3+ and P5+ ion contents. The decrease of cooperative luminescence of Yb3+ ion pairs indicates a good dispersion effect of Al3+ and P5+ ions on Yb3+ ions in Yb3+-doped silica glass. The results are useful for optimization of fabrication of the high quality Yb3+-doped silica fiber by the composite design of Yb–Al–P co-doped silica glass.

2 μm laser properties of Tm 3+ -doped large core sol-gel silica fiber

Tm3+-doped 0.1Tm2O3-1Al2O3-98.9SiO2 (mol%) silica glass with good uniformity was prepared by sol-gel method combining with high temperature sintering. The core glass sized Φ3.2 × 50 mm with Δn of 5 × 10−4 was obtained after gelation, heat treatment, melting and polishing. Its spectroscopic properties were evaluated according to the detected absorption and fluorescence spectra. The maximum emission cross-section of Tm3+ ion in this glass is 6.2 × 10−21 cm2 and tested fluorescence lifetime is 836 μs at 1806 nm. Large core double cladding fiber with core NA of 0.102 was prepared by rod-in-tube and high temperature drawing. Its tested optical loss is 1.1 dB/m at 1333 nm. 1.11 W fiber laser output centered at 1969 nm with M2 factor of 1.99 was obtained from a 140 cm length double-cladding fiber with core diameter of 38 μm. The quasi-single mode laser with M2 factor of 1.33 was achieved in the fiber with core diameter of 19 μm.

Spectroscopic and laser properties of Al-P co-doped Yb silica fiber core-glass rod and large mode area fiber prepared by sol-gel method

A large-size (∅5 mm) Yb3+-doped silica fiber core-glass rod with an Al-P co-doped composition: 0.035Yb2O3-1.0Al2O3-1.0P2O5-97.965SiO2 (mol%) was prepared by the sol-gel method combined with high-temperature melting technology. We successfully solved the doping homogeneity problem caused by the volatility of P2O5. The doping homogeneity of the glass rod was very high with the maximum refractive index fluctuation Δn < 2 × 10−4. The refractive index of the glass rod was very low because of the equimolar amounts of Al and P co-doping, which yielded an AlPO4 structure. A large-mode-area single cladding fiber (LMA SCF) and photonic crystal fiber (LMA PCF) with core diameters of 35 and 50 µm, respectively, were drawn from this glass rod. Owing to the low refractive index of the core-glass rod and the corresponding low numerical aperture (NA, 0.033) of the core, the LMA SCF exhibited a high laser beam brightness with M2 = 1.3-1.4. The LMA PCF exhibited the maximum output power, which was limited by the available pumping power, of 46 W and the slope efficiency of 61%.

Yb^3+-doped silica glass rod with high optical quality and low optical attenuation prepared by modified sol-gel technology for large mode area fiber

We report on a modified sol-gel method combined with an innovative high-temperature melting technology for the preparation of Yb3+-doped silica glass rods (circle divide 3-18 mm) with high optical quality and low background loss. We prepared Al-Yb, Al-P-Yb, Al-FYb, and Al-P-F-Yb doped silica glass rods for large mode area fibers (LMA) with a high laser power and low core numerical aperture (0.02). We were able to successfully solve the doping homogeneity problem caused by the volatility of P and F. More importantly, we developed a purification technology and successfully reduced the optical attenuation to 0.05 dB/m. An Al-Yb co-doped silica photonic crystal fiber (PCF) with a core diameter of 100 mu m was fabricated for laser behaviour characterization. In the continuous wavelength laser measurement, a laser output slope efficiency of 83.3% was obtained from the fabricated PCF. To our knowledge, this is the highest slope efficiency derived from a Yb3+-doped silica PCF prepared by a non-CVD method. In the pulse amplification laser experiment, an average amplified power of 310 W with a peak power of 1.5 MW and a pulse duration of 21 ps were achieved.